The Channel tunnel rail link and the British end of Eurostar were effectively "nationalised" yesterday through a ruling by the Office of National Statistics that they are under the control of the government.

The ONS said the parent company of the two businesses, London & Continental Railways, has been reclassified as a public non-financial corporation - similar in status to Royal Mail and London Underground - because of the government's influence on its policy and purse.

The decision adds £5bn to the Treasury's balance sheet and takes national debt to £443bn. However, it means the government is free to decide on whether a takeover bid for LCR by the City grandee Sir Adrian Montague can go ahead - irrespective of the views of management or private-sector shareholders.

After a lengthy investigation, the ONS disclosed that a government guarantee for £3.7bn of LCR's borrowing gives ministers the power to force a sale of the business and pocket 90% of the proceeds. Ministers can institute boardroom changes and claw back a share of LCR's cashflow.

Crucially, financial covenants show that the government can claim 50% of the profit on potentially lucrative development of the 120 acres of land owned by the company near Stratford station, east London, which includes part of the proposed site of the 2012 Olympic village.

The decision might have implications for Sir Adrian's plans for a takeover. He is chairman of British Energy and has close links with the government, having chaired an industry taskforce for the Treasury.

Insiders suggested it "added complexity" - not least because Sir Adrian is now a public-sector employee attempting to buy another public-sector entity.

A spokesman for Sir Adrian's bid team would only say: "We will carefully consider the implications of this announcement for our ongoing interest in acquiring LCR."

The ONS said it had been asked by the Treasury last year to judge whether a securitised bond issue by LCR should be counted as public debt. The ONS decided it should and that prompted statisticians to take a fresh look at the extent to which LCR was controlled by the government. "We are concerned with control rather than ownership," said a statistician, adding that the change was effective back to 1999.

The ruling means that the government ultimately owns 11 of Eurostar's 31 trains, which are part of the British arm of the cross-Channel services. Ticket office and sales staff on this side of the Channel become public-sector workers. So does LCR's chief executive, Rob Holden, who is now one of the highest-paid employees on the public payroll with a salary of £358,000 plus undisclosed bonuses.

Mr Holden opposes a takeover - he wants to break up LCR. LCR's shareholders include National Express, UBS, Bechtel and Electricité de France - several of which are looking for an exit.

For the chancellor, the main impact is that implied interest payments of about £200m a year are added into the public finances, making it slightly more difficult for him to meet his self-imposed "golden rule" to balance the budget over the economic cycle.